i have the same scenario with morris/mathews in a league, except that my other RB is shadeball mccoy.... and i've stuck with morris. he's been nice and consistent and even tho RG3 is a point sponge (and of course, i'm up against him) i'm really hoping morris goes off, even tho rg3 is a short pass master and he gets a lot of those rushing yards.
as for chris johnson.... welp, you know what you're in for. he's definitely got more HR/big-game threat and as we know his downside is very there.
i'd look at it like this.... depending on your matchup, if you're looking at a close one where you need for-sure 10pts from each RB, go with morris and mathews. if you need that chance of a big week at RB to have a chance in a matchup where you're like "oh [fecal matter]" and you need that 20pts/+ upside, then go with chris johnson, noting the chance he could go splat and get you 4-6pts.
so yeah, definitely morris and then if you want a higher shot at a "guaranteed" ~8-10pts i'd go matthews... if you wanna take the risk to have a 20+pts game, then go with CJ. thx for helping out on mine